Over the years, the Ethernet has become the most commonly used method for local area networking. PoE and PoE+ systems provide power delivery over unshielded twisted-pair wiring from Power Sourcing Equipment (PSE) to a PD located at opposite sides of the Ethernet link. Powered Devices (PDs) may include such network devices as IP phones, wireless LAN access points, personal computers, Web and security cameras, etc. The PoE/PoE+ system supports providing power to PDs over Ethernet cabling used for data transmission.
The PoE system is defined in the IEEE 802.3af standard, and the PoE+ system is described in the IEEE 802.3at draft standard. PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device.
PSE searches the link for a PD requesting power, optionally classifies the PD, supplies power to the link if a PD is detected, monitors the power on the link, and disconnects power when it is no longer requested or required. PD participates in the PD detection procedure by presenting a PoE detection signature defined in the IEEE 802.3af standard and/or the IEEE 802.3at standard (referred to below as “IEEE 802.3af/IEEE 802.3at standard”). If the detection signature is valid, the PD has an option of presenting to the PSE a class signature defined in the IEEE 802.3af/IEEE 802.3at standard, to indicate how much power it will draw when powered up. Based on the determined class of the PD, the PSE applies the required power to the PD.
In addition to a PoE/PoE+ power supply input coupled to an Ethernet link, the PD may have one or more auxiliary power supply inputs for receiving power from auxiliary power supply sources to support operation of the PD when the power from the PSE is not available.
Therefore, it would be desirable to develop efficient and cost-effective circuitry that would enable a PD to receive power from the PoE/PoE+ power supply input and from the auxiliary power supply inputs, and provide seamless switching among various power supply inputs.